1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method that control an image clock frequency (hereinafter referred to as the “laser driving frequency”) used in ON/OFF control of a laser beam that scans an image carrier such as a photosensitive member.
2. Description of the Related Art
In an electrophotographic image forming apparatus, generally, latent image formation on a photosensitive member is carried out by scanning a laser beam emitted from a semiconductor laser by turning the laser beam on and off using a polygon mirror such as a photosensitive drum so that the laser beam is irradiated onto the photosensitive member, developing the latent image into a toner image, and transferring the toner image onto a recording medium.
A conventional optical scanner unit is shown in FIG. 4.
In the conventional optical scanner unit, a laser beam is emitted from a laser beam source 101 based on an image clock signal determined by the resolution of an image to be formed, and the emitted laser beam is deflected by a polygon mirror 102 rotating at a constant angular velocity and irradiated onto a photosensitive member 105 via a f-θ lens 103 and a reflecting mirror 104.
At this time, due to the action of the polygon mirror 102 rotating at a constant angular velocity and the f-θ lens 103, the laser beam is irradiated onto the photosensitive member 105 while being moved in a direction indicated by the arrow (in the main scanning direction) at a constant velocity. The polygon mirror 102 is disposed at a location opposite a central portion of the photosensitive member 105, and therefore, if there were no f-θ lens 103, the length of the optical path of the laser beam would be greater at end portions of the photosensitive member 105 than at the central portion thereof, with the result that the main scanning speed would be faster at the end portions of the photosensitive member 105 than at the central portion thereof.
Thus, the f-θ lens 103 acts to correct differences in the main scanning speed with respect to the photosensitive member 105 due to the differences in the length of the optical path to the photosensitive member 105 of the laser beam deflected by the polygon mirror 102, so as to make the main scanning speed constant between the end portions of the photosensitive member 105 and the central portion thereof.
However, it is difficult to maintain the laser beam main scanning speed constant with respect to the photosensitive member 105, due to such factors as variations in the characteristics of the f-θ lens 103, variations in the laser beam wavelength, irregularities in the rotational speed of the polygon mirror 102, and fluctuations in the characteristics of the f-θ lens 103 due to changes in ambient temperature, so that the positions of dots of an electrostatic latent image formed on the photosensitive member 105 deviate slightly, thus leading to distortion of the image, color shift, color irregularities, and so forth.
Methods of solving the above-described problem have been proposed, which include a method of driving the laser beam at a periodically varying frequency according to the characteristics of the f-θ lens, and a method of detecting a position on the photosensitive member at which the main scanning is completed, and uniformly modulating the laser driving frequency according to the deviation of the scanning completion timing from a desired timing.
However, although according to the above-described methods, values of the main scanning speed at positions on the main scan line can be averaged to obtain a desired main scanning speed, the main scanning speed cannot be arbitrarily varied for each position on the main scan line, and therefore, the dots of the electrostatic latent image cannot be formed at appropriate positions in terms of units of pixels.
Moreover, there has been employed a technique of modulating the frequency of an image write clock so that a printing position is electrically corrected. For example, there are a method of varying the frequency uniformly over a single scanning section, and a method of dividing a single scanning section into a plurality of divided sections, and analogically varying the image write clock frequency for each of the divided sections (for example, Japanese Laid-Open Patent Publication (Kokai) No. H02-282763.)
However, in these methods that modulate or vary the laser driving frequency, the frequency is also modulated from a main scanning starting position to an image write position, and therefore the image write position may shift.